This invention relates to improvements in or relating to processes involving the interaction between a gas and the surface of a catalyst to regenerate the catalyst. The application is related to our copending application filed on even date herewith and entitled "Improvements in the Conversion of Chemical Moieties".

Catalyst regeneration is an important aspect of many commercial chemical syntheses. In a wide variety of chemical reactions involving the use of catalysts, the activity of the catalyst decreases with time due to poisoning of the catalyst or to modification of the catalyst surface. As a result, conversion, selectivity and/or reaction rate deteriorate and regeneration of the catalyst is therefore important from an economic viewpoint.

In general, regeneration is effected by washing or by heating in the presence of a suitable gas. However, the procedures are complicated or expensive or both and in general the activity of the regenerated catalyst is not as high as that of the freshly made material. Thus, each time the catalyst is poisoned and regenerated, its activity is further reduced until it is no longer effective as a catalyst.

Japanese patent 51088490 describes a method of regenerating platinum catalyst by glow discharge at 5kV DC in 10 " ² torr Argon for 30 minutes. A disadvantage of this method is that the electrodes have to be situated within the regeneration reaction chamber where they may be damaged due to the corrosive nature of the reaction. This can lead to further contamination of the catalyst. Further, the electrodes may themselves be contaminated by the action of the gases produced during the regeneration procedure, thus reducing their efficiency.

VJe have now found and alternative method of regenerating catalysts which enables the above-mentioned risks to be reduced or avoided. It has also been observed to produce a regenerated catalyst that has an activity substantially equal to or even better than that of fresh catalyst.

In accordance with the present invention, there is provided an improvement in the method for the regeneration of a solid catalyst which involves contacting the surface of the catalyst with a gas, wherein the gas is provided in the form of AC excited plasma.

In general, the process of regeneration will involve the removal from the catalyst surface of a material or materials that have poisoned the surface. Where the poison is adbsorbed on the surface, the process may comprise physical removal of the poison, either as such or by chemically first converting it to another form, e.g. a gas, which is more readily removable. In some cases, however, the poison may have combined chemically with the surface, in which case the process will involve breaking the chemical bonds.

In some regeneration procedures according to the present invention, the catalyst surface itself may be adversely affected and in such cases the procedure may include a step of restoring the surface to the desired form by the action of plasma. This restoration step may involve a physical modification, a chemical modification or both.

The invention also provides apparatus suitable for use in the process, said apparatus comprising a zone containing solid catalyst material and a plasma-forming zone, means for providing gas to the plasma-forming zone, electromagnetic coupling means for generating an ionising AC electric field in the plasma-forming zone to form plasma from the gas, and means for contacting the catalyst with

the plasma .

In one alternative, the plasma-forming zone and the zone containing the catalyst may be the same zone. In an alternative embodiment, the plasma-forming zone and the catalyst-containing zone are different and means are provided for transferring plasma formed in the first mentioned zone to the second.

An advantage of the invention is that the electromagnetic coupling means, e.g. inductive coil or capacitative plates, may be located outside the vessel thus eliminating the risk of cross-contamination between the electromagnetic coupling means and the catalyst.

The invention, which will first be described with the reference to its application to the regeneration of poisoned catalyst, is now described by way of example with reference to the accompanying drawings in which:

Figure 1 is a schematic drawing of one apparatus suitable ^" for use in the invention, and

Figure 2 is a graph comparing the activity of fresh catalyst, the same catalyst after poisoning and the poisoned catalyst regenerated by the process of the invention.

Referring to Figure 1, the apparatus comprises a vessel (1) having an inlet (2) through which gas may by introduced into the vessel and an outlet (3) connected to a pump, not shown. A further outlet (4) is connected to a Pirani gauge, not shown. A flange (5) allows the transfer of catalyst into and out of the vessel (2) . A coil (6) is wound around the vessel (1) and connected to an alternating current source via a matching network. This serves to match the external load to the output impedance of the

generator, thus ensuring maximum power.

In an alternative embodiment, not shown, the coil (6) may be replaced by the plates of a capacitor which are situated on opposed sides of the vessel (1).-* It will be clear that different designs of inductively coupled coils or capacitatively coupled plates can be arranged to suit the reactor design in order to maximise the power transfer.

In a typical operation, the catalyst is located in the vessel and the gas is introduced at low pressure and converted to plasma by excitation by means of an AC electric field. The catalyst may be located within the field or outside the field, e.g. by providing the vessel in the form of tubular reactor through which the gas is caused to pass from a plasma-forming zone to a downstream zone containing the catalyst. In an alternative embodiment, not shown, the apparatus may consist of two vessels, the gas being excited to a plasma by an AC electric field in the first vessel prior to being passed to the second vessel containing the catalyst. The second vessel may operate under different conditions of temperature and/or pressure to those of the first vessel.

The alternating current employed for generating the plasma preferably has a frequency of l-10 ¹² Hz, more preferably 10 ³ - 10 ⁹ Hz. It will be understood, however that in some countries the frequencies that may be used are limited, e.g. because of the risk of interference with radio transmissions. For example, in Great Britain, a frequency of 13.56MHz is set aside by the Government for such experiments and will not therefore interfere with radio transmissions. Other frequencies can be used provided that the Government is advised of the intention to use these frequencies.

Frequencies of less than 1 Hz may also be used. However,

such frequencies would give rise to alternating or periodic glow discharge rather than a continuous plasma. Such discharges are advantageous for example, when the power input has to be minimised or for providing additional control of the reaction at the catalyst surface.

The plasma is normally generated from gases at sub- atmospheric pressure. Pressures of from 100 to 10" ³ Torr are generally suitable. However, the pressure used is dependent on the power loadings. Therefore, if a sufficiently high power loading is available, it may be possible to excite gas to plasma at a pressure above 100 Torr, if that is desired.

The plasma may also be generated from liquids, for example plasmas of hydrogen and oxygen can be generated from water.

Catalyst which may be regenerated by the process of the present invention are, for example, metallic catalysts such as iron, nickel, palladium, platinum, iridium, rhodium, ruthenium, osmium and silver or any catalyst comprising an element from Group IV, V, VI, VII, VIII, lb, lib, Ha and Ilia of the Periodic Table including those containing additional promoters such as oxides from Group la which may be used in hydrogenation, dehydrogenation, hydrolysis and oxidation reactions; semiconducting oxide and sulphide catalysts such as NiO, ZnO, Mn0 ₂ , Cr ₂ 0 ₃ , Bi ₂ 0 ₃ , Mo0 ₃ and WS ₂ which may be used in oxidation, dehydrogenation and desulphurisation and hydrogenation reactions and insulator oxide catalysts such as A1 ₂ 0 ₃ , Si0 ₂ and MgO which may be used in dehydration reactions.

The catalyst, which may be supported if desired, may be of any suitable shape.

The choice of gas for use as the AC plasma will depend on the nature of the poison on the catalyst. Examples of such

gases are oxygen, hydrogen, hydrogen fluoride, fluorine, carbon tetrafluoride vapour, krypton, argon, neon and helium.

For example, silicon and silicon oxides and nitrides can be removed with fluorine species, phosphorus can be removed by hydrogen plasma, hydrocarbons can be removed in an oxygen plasma while sulphur and its different forms can be removed by sequential treatment with plasmas of oxygen and hydrogen. In each of these cases, the poison is converted by reaction with the plasma into volatile products which can readily be removed from the reaction zone.

Gases may be used in admixture e.g. to remove more than one poison. For example, carbon and silicon can be removed simultaneously in plasmas of oxygen and fluorine. It is also possible to remove more than one poison by treating the catalyst with the plasmas of suitable gases sequentially. For example, sequential treatment with oxygen and hydrogen plasmas can be used to remove a wider range of compounds including carbon, sulphur and nitrogen.

Where the product of the reaction between the poison and the plasma is other than gaseous, a liquid, e.g. in the form of a thin film or an aerosol, may be passed over the surface of the catalyst to remove the product from the catalyst site. Where the product is a solid, the chosen liquid is in one preferred embodiment, one in which the solid isoluble.

In the above-described embodiments, the removal of the poison involves a chemical conversion step. However, it is also envisaged that where the poison is adsorbed on the catalyst surface, it may be removed by a procedure which involves physically dislodging it be means of the plasma.

Where the catalyst has been poisoned or deactivated due to

physical changes in the microstructure. The plasma may be used to restore the physical structure of the catalyst.

Certain catalysts, for example palladium catalysts, are commonlly chlorinated e.g. to extend their shelf life and selectivity. Where, after such catalysts have become poisoned, they are regenerated by the process of the present invention, the regeneration process may also dechlorinate the surface of the catalyst thereby reducing its shelf life and selectivity. In such cases, the process of the invention may also be employed restore the catalyst surface, e.g. by oxidation or reduction reactions with plasma gas. For example, in the specific example of surface-chlorinated palladium, the catalyst, after treatment to remove the poison, may be treated with chlorine plasma to rechlorinate the surface of the catalyst if desired followed by reduction in hydrogen plasma to control the final level of chlorination. _ Chloroform, carbon tetrachloride vapour and gaseous halogenated hydrocarbons, e.g. Freons, can be used to achieve the same effect as the chlorine. Other surface features adversely affected during regeneration of the catalyst may be restored by treatment with an appropriate plasma.

By means of the invention a catalytic treatment of internal combustion engine exhaust gases can be envisaged in which a component of the exhaust gas is converted to plasma for the purpose of surface modifying or regenerating the catalyst.

It will be appreciated that where the catalyst contains a conductive component, e.g. a metal, the alternating current will induce eddy currents in the catalyst and result in the generation of heat. This heating can assist the dynamics of the regeneration or modification reaction by providing thermal energy. If, however, heating is not required, cooling may be provided.

The effect of eddy currents in the catalyst can be reduced by using a lower frequency to generate the plasma as induced current is proportional to (frequency) ² for a given conducting particle size. Thus the frequency of the alternating element can be adjusted to control the temperature and heating of the catalyst surface. The use of a lower frequency has additional advantages in that it is easier to generate the plasma and manipulate the electrical signals. The ability to more easily manipulate the electrical signals means that cheaper more readily available components can be used.

An additional problem with metallic catalysts is that the particles of the catalyst may scatter the electric field when the wavelength of the alternating field approaches the size of the catalyst particles. This problem may be reduced or obviated by an appropriate choice of wavelength.

Where the reaction vessel is large, as in an industrial scale reaction, it is preferable to generate the plasma at lower frequencies, such as 40kHz, as the plasma is less likely to vary in intensity across the vessel.

If higher frequencies are used, nodes and antinodes of plasma intensity may be created which may result in power loss and a reduction in the efficiency of the process.

The invention will now be illustrated with reference to specific examples. However, it is to be understood that the invention is not limited by these examples.

Example 1

A 0.5% palladium on alumina catalyst from Englehard Industries, which had been poisoned by exposure to hydrogen sulphide gas for 48 hours, was centrally located in the cavity (1) of the reactor configuration of Figure 1. The reactor was then connected to a vacuum line of grease-free

construction pumped by a 100 litre min" ¹ two-stage rotary pump. Pressure was monitored by a Piraήi gauge and the gases from which the plasmas were to be formed were introduced sequentially into the system via a leak valve at the required pressure (0.15mbar). A liquid nitrogen trap was incorporated into the system to prevent back streaming of rotary pump oil and to assist low pressure by collecting the vapour in a trap. The reactor was evacuated to 10" ³ mbar.

Plasmas of hydrogen and oxygen were excited by an ENI 13.56MHz radio-frequency generator (Model ACG3) capable of delivering a power output to 350 watts in continuous mode. The generator was matched to the external inductive load via an impedance network. This network served to match the external load to the output impedance of the generator, thus ensuring maximum power transfer to the external circuit and to protect the generator.

The plasmas were generated at 80 watts. Regeneration of the catalyst was achieved by 15 minutes exposure to each of the hydrogen and oxygen plasmas sequentially at 0.15m bar pressure. Badly poisoned catalyst, however, may require several hours to regenerate.

Example 2

The catalytic activities of O.lg of the regenerated catalyst of Example 1, O.lg of a fresh sample of the same 0.5% palladium on alumina catalyst as used in Example 1 and O.lg of the poisoned catalyst of Example 1 were assessed in a solution phase. The rate of consumption of hydrogen in its reaction with 0.3cm ³ ethyl crotonate in 15cm ³ ethylacetate in the presence of the catalyst at standard temperature and pressure was measured as this gives a direct corrolation with catalyst activity.

The results of the experiments are shown graphically in

Figure 2. It is clear from this graph that the catalysing activity of regenerated catalyst is better than that of fresh catalyst.

Example 3

A 0.5% palladium on alumina catalyst from Englehard Industries, which has been poisoned by exposure to dry ammonia gas for 48 hours, was centrally located in the cavity (1) of the reactor configuration of Figure 1. The reactor was then evacuated using a 100 litre min" ¹ two-stage rotary pump. Pressure was monitored by a Pirani gauge and the gauge and the gas from which the plasma was to be formed was introduced via a leak valve at the required pressure (0.1 Torr). A liquid nitrogen trap was incorporated into the system to prevent back streaming of rotary pump oil and to assist low pressure by collecting the vapour in a trap. The reactor was evacuated to 10" ³ mbar.

A plasma of oxygen was excited by an ENI 13.56MHz radio- frequency generator (Model ACG3) capable of delivering a power output to 350 watts in continuous mode.

The plasma was generated at 100 watts. The catalyst was exposed to the plasma for 15 minutes.

Example 4

Example 3 was repeated except that the poisoned catalyst was exposed to the plasma for 30 minutes.

Example 5

The catalytic activities of 0.27 of the regenerated catalyst from Example 3, 0.27g of the regenerated catalst from Example 4; 0.27g of a fresh sample of the same 0.5% paladium on alumina catalyst as used in Examples 3 and 4 and 0.27g of poisoned catalyst were assessed in a solution phase. The rate of consumption of hydrogen in its reaction

with 0.4 cm ³ of ethyl crotonate in 10cm ³ ethanol in the presence of the catalyst at standard temperature and pressure was measured.

The activity of the regenerated catalysts from Examples 3 and 4 was better than that of the poisoned catalyst. The activity in each case was 55% of the fresh catalyst. No difference in activity was observed between the regenerated catalysts of Examples 3 and 4.